1. Field of the Invention
The present invention relates to an image reading apparatus, and particularly to an image or color picture image reading apparatus wherein there are provided an image-forming optical element or device having different refracting powers with respect to a main scanning direction and a sub-scanning directions, a color separating means comprised of a reflection type one-dimensional blazed diffraction grating having a predetermined shape and a light receiving means of a predetermined shape in which three line sensors or sensor arrays (light receiving elements) are arranged on a common substrate. The light receiving elements are arranged for preventing the occurrence of blurring of images of respective color light components formed on the sensor arrays due to the deviation of a reflective diffraction angle resulting from the difference in an incident angle on the blazed diffraction grating and hence color picture image information on an object or an original surface can be read with high precision. The image reading apparatus of the present invention can preferably be used in, for example, a color scanner and a color facsimile.
2. Related Background Art
Conventionally, there have been proposed various kinds of apparatuses for forming the images of a color picture image information on an object on respective line sensor (CCD) surfaces through an optical system and digitally reading the color picture image information by using an output signal from the line sensor.
FIG. 1 schematically shows the main part of an optimal system of a prior art color image reading apparatus. In FIG. 1, a light beam from a color picture image on an object surface 11 is condensed by an image-forming lens 19 to be imaged on line sensor surfaces (described below). In this structure, the light beam is color-separated into three light color components such as red (R), green (G) and blue (B) by a three-piece (3P) prism 20 and thereafter the separated color components are directed onto the respective line sensors 21, 22 and 23. A line scanning in an auxiliary scanning direction is performed for the color picture images formed on the respective line sensors 21, 22 and 23 to achieve an image reading of the respective color light components.
FIG. 2 schematically shows the main part of an optical system of another prior art color image reading apparatus. In FIG. 2, a light beam from a color picture image on an object surface 11 is condensed by an image-forming lens 29 to be imaged on line sensor surfaces explained below). In this structure, the light beam is color-separated into three light beams corresponding to three light color components by two color-separating beam splitters 30 and 31 which respectively have wavelength-selective dichroic transmission films. The color picture images corresponding to three color light components are respectively formed on a so-called monolithic three-line sensor 32 having three line sensors arranged on a common substrate. Line scanning in a sub-scanning direction is performed for the color picture images to achieve an image reading of the respective color light components.
FIG. 3(a) shows the monolithic three-line sensor 32 illustrated in FIG. 2 and FIG. 3(b) is an enlarged view of section 28 of line sensor 25. In the monolithic three-line sensor 32, three line sensors (CCD) 25, 26 and 27 are arranged in parallel equal distances apart from one another on a common substrate, and color filters (not shown) corresponding to the respective color light components are provided on the respective line sensor surfaces
Distances S.sub.1 and S.sub.2 of the line sensors 25, 26 and 27 are generally set to, for example, approx. 0.1-0.2 mm, and pixel widths W.sub.1 and W.sub.2 of each single element of line sensors 25, 26 and 27 are set to, for example, approx. 7 .mu.m.times.7 .mu.m and 10 .mu.m.times.10 .mu.m, under various manufacturing conditions.
The color picture image reading apparatus shown in FIG. 1, however, requires three independent line sensors 21, 22 and 23 and a highly precise structure, as well as the 3P prism 20 which is difficult to manufacture. Hence, the entire apparatus becomes complicated and expensive. Furthermore, the alignment adjustment between the image-forming light beam and each line sensor should be conducted independently for the respective line sensors 21, 22 and 23, and hence, the assembly adjustment is cumbersome.
Further, in the color picture image reading apparatus shown in FIG. 2, the distance between the lines of the line sensors becomes 2.sqroot. 2X when the plate thickness of the beam splitters 30 and 31 is set to a value of X.
Assuming the distances between the lines of the line sensors that are preferable in their manufacture are equal to approx. 0.1-0.2 mm, the plate thickness X of the beam splitters 30 and 31 becomes approx. 35-70 m.
Generally, it is considerably difficult to structure a beam splitter having such a thin thickness while maintaining an optimum optical flatness. As a result, degradation of the optical performance of the color picture image formed on the line sensor occurs when the beam splitter having such a thickness is used.
On the other hand, as shown in FIGS. 4(a) and 4(b), the distances S.sub.1 and S.sub.2 from the center line 26 of the monolithic three line sensor to the other two lines 25 and 27, respectively, are generally equal to each other in opposite directions, and this distance is set to an integer multiple of the pixel size W.sub.2 (see FIG. 3) in the sub-scanning direction. The reason therefor is as follows:
When the read-out of the color picture image is performed by the above-described monolithic three line sensor using only an ordinary image-forming optical system 43, as shown in FIG. 4, read-out positions on the object surface 11 which can simultaneously be read by the three line sensors 25, 26 and 27 are three different positions 25', 26' and 27' as shown in FIG. 4.
Therefore, respective signal components of three colors (R, G, B) for any one position on the object surface 11 cannot be read simultaneously, and instead, after the respective read-outs by the three line sensors, the read-out color component signals for the same position on the object surface 11 must be combined.
For this purpose, the distances S.sub.1 and S.sub.2 between the lines of the three line sensor are set to an integer multiple of the pixel size W.sub.2, and corresponding thereto, a redundant line memory is provided. In this structure, the combined signal of the three color signal components for the same position on the object can be readily obtained by, for example, delaying the G and R signals (signal components corresponding to the G and R color light components) relative to the B signal (a signal component corresponding to the B color light component).
Thus, the distances S.sub.1 and S.sub.2 from the center line 26 of the monolithic three line sensor to the other two lines 25 and 27 are set to integral multiples of the pixel size W.sub.2 in the sub-scanning direction.
In the above-discussed color picture image reading apparatus, however, the redundant line memory should be fully arranged between the lines of the three line sensor, and hence a plurality of expensive line memories should be provided. As a result, the apparatus has a high manufacturing cost and the structure of the apparatus becomes complicated.
Further, a color picture image reading apparatus is known in which a blazed diffraction grating is employed instead of a dichroic mirror (see U.S. Pat. No. 4,277,138 issued to Hans Dammann on Jul. 7, 1981). In this structure, there is provided an optical system in which a blazed diffraction grating is used as a color separating means.
This structure, however, is subject to the blur of images of respective color light components due to the deviation of a reflective diffraction angle resulting from the difference in an incident angle on the blazed diffraction grating.